1. Field of the Invention
This invention relates to infrared detectors and, more specifically, to an infrared detector responsive to infrared energy in two different spectral bands.
2. Brief Description of the Prior Art
Infrared energy is of most interest in the spectral bands of 3 to 5 micrometers and 8 to 14 micrometers. In the present state of the art, infrared focal plane arrays are used to detect infrared radiation in only one spectral band, usually either the 3 to 5 micrometer or the 8 to 14 micrometer band. Multi-spectral band detectors offer the advantage of meeting a wide range of spectral band requirements for different tactical and strategic scenarios with the same detector. In addition, such detectors can be used for target recognition by simultaneously detecting the scene being scanned in two spectral bands and using the results to determine the temperature of the scene being scanned.
In the present state of the art, there are two principal types of HgCdTe two-color detectors, these being the MIS heterojunction detector and the triple layer heterojunction diode. Due to their unique material requirements, however, these types of detectors are generally very difficult to fabricate. It is therefore a desire of the art to overcome the material requirement inherent in the prior art HgCdTe detectors.
The HgCdTe MIS heterostructure concept has been demonstrated with molecular beam epitaxy (MBE) grown heterostructures. Devices of this type are set forth in articles of M. W. Goodwin et al., Proceedings of the IRIS Detectors Specialty Group, Monterey, Calif., 1989 and M. W. Goodwin et al, Journal of Vacuum Science Technology, A8, page 1226, 1990. The structure is basically an MIS device including a thin wide bandgap N-type layer over a thick N-type narrow bandgap layer. The thickness of the wide bandgap layer is on the order of the depletion region beneath the gate of the MIS device. With the proper dimensions of layer thickness, determined by the carrier concentration, the structure can detect radiation consistent with the wide bandgap layer or wide plus narrow bandgap layer, depending upon the appropriate voltage thereacross. The disadvantages of this structure as a two-color (two spectral band) detector are that it requires precise control of both the layer thickness and the carrier concentration and is not a true two-color detector in the sense that it detects separately narrow and wide bandgap radiation.
The triple layer heterostructure diode has also been demonstrated on a hybrid liquid phase epitaxy (LPE)-MBE grown heterostructure. Devices of this type are set forth in an article of T. N. Casselman et al., Proceedings of the IRIS Detectors Specialty Group, Gaithersburg, Md., 1990. The structure includes back-to-back diodes with the center layer composed of a short wave P-type layer. The two N-type regions of the diodes serve as the detecting layers (the wide and narrow bandgap layers). Although the wide bandgap and short wave layers are grown by LPE, the top narrow-bandgap layer must be grown by MBE due to severe grading problems with LPE. In addition, precise control of the placement of the N-P junctions is necessary for low dark currents.
In addition to HgCdTe devices, vertically stacked group III-V quantum wells have also been proposed as possible multi-spectral detectors. Devices of this type are shown in an article of B. F. Levine et al., Applied Physics Letters, volume 53, page 2196 (1988).